Ass 3: Q3 it actually works

1 files changed, 123 insertions, 0 deletions

diff --git a/ass3/q3/decoder_lzss.py b/ass3/q3/decoder_lzss.py
new file mode 100644
index 0000000..d7dcf0c
--- /dev/null
+++ b/ass3/q3/decoder_lzss.py
@@ -0,0 +1,123 @@
+import sys
+
+
+class Stream:
+    def __init__(self, string):
+        self.string = string
+        self.cursor = 0
+
+    def rewind(self):
+        self.cursor = 0
+
+    def end_of_stream(self):
+        return self.cursor >= len(self.string)
+
+    def read_elias(self):
+        offset = self.cursor
+        if self.string[0 + offset] == "1":
+            self.cursor += 1
+            return 1
+        start, end = 1, 2
+        while True:
+            binary = self.string[start + offset:end + 1 + offset]
+            if binary[0] == "0":
+                chunk = int(f"1{binary[1:]}", 2) + 1
+                start = end + 1
+                end += chunk
+            else:
+                self.cursor += end + 1
+                return int(binary, 2)
+
+    def read_binary(self, length):
+        blob = self.string[self.cursor:self.cursor + length]
+        self.cursor += length
+        return blob
+
+    def read_int(self, bits):
+        return int(self.read_binary(bits), 2)
+
+    def read_char(self, bits=7):
+        return chr(self.read_int(bits))
+
+    def read_bit(self):
+        return self.read_binary(1)
+
+    def read_huff_char(self, decode_table):
+        window_size = 1
+        window = self.string[self.cursor:self.cursor + window_size]
+        while window not in decode_table:
+            if window_size >= len(self.string):
+                raise Exception("Went for too long looking for a Huffman char")
+            window_size += 1
+            window = self.string[self.cursor:self.cursor + window_size]
+        self.cursor += window_size
+        return decode_table[window]
+
+    def read_lzss_tuple(self, decode_table):
+        format_type = self.read_bit()
+        if format_type == "0":
+            return 0, self.read_elias(), self.read_elias()
+        else:
+            return 1, self.read_huff_char(decode_table)
+
+
+def elias(n):
+    assert n > 0
+    binary = bin(n)[2:]
+    result = binary
+    while len(binary) != 1:
+        binary = "0" + bin(len(binary) - 1)[3:]
+        result = binary + result
+    return result
+
+
+def decode_elias(encoded, offset=0):
+    if encoded[0 + offset] == "1":
+        return 1
+    start, end = 1, 2
+    while True:
+        binary = encoded[start + offset:end + 1 + offset]
+        if binary[0] == "0":
+            chunk = int(f"1{binary[1:]}", 2) + 1
+            start = end + 1
+            end += chunk
+        else:
+            return int(binary, 2)
+
+
+def parse_header(stream):
+    huff = {}
+    alphabet_size = stream.read_elias()
+    for i in range(alphabet_size):
+        char = stream.read_char()
+        length = stream.read_elias()
+        codeword = stream.read_binary(length)
+        huff[codeword] = char
+    return huff
+
+
+def parse_body_tuples(stream, decode_table):
+    num_tuples = stream.read_elias()
+    return [stream.read_lzss_tuple(decode_table) for _ in range(num_tuples)]
+
+
+def decode_lzss(tuples):
+    result = ""
+    for tup in tuples:
+        if tup[0] == 0:
+            _, offset, length = tup
+            cursor = len(result) - offset
+            for i in map(lambda x: x % offset, range(length)):
+                result += result[cursor + i]
+        else:
+            result += tup[1]
+    return result
+
+
+def decode_message(raw_data):
+    stream = Stream(raw_data)
+    decode_table = parse_header(stream)
+    tuples = parse_body_tuples(stream, decode_table)
+    return decode_lzss(tuples)
+
+print(decode_message("01111000011111000100100011000110100100011111111010011000100100001101111"))